Forest Litter Decomposition And Soil Respiration Characteristics In Baotianman

Global warming is the most serious environmental problem threatening human survival, and the heavy emissions of greenhouse gases are considered the main causes of global warming. Litter decomposition and soil respiration as two forms of carbon fluxes have significant implications to global climate change. However, there are many uncertainties and debate on the underlying mechanisms and controlling factors of litter decomposition and soil respiration. This study investigated foliar litter decomposition and soil respiration dynamics across contrasting forest stands in central China, including pure stands of Quercus aliena var. acuteserrata, Quercus glandulifera var. brevipetiolata, and Quercus variabilis, respectively, and mixed pine/oak stands dominated by Pinus armandii and Q. aliena var. acuteserrata, as well as stands of pure Q. aliena var. acuteserrata trees ranging in stand age from～40 to>160 years. Litter decomposition was studied by using litterbag method and soil respiration was measured with a Li-Cor infrared gas analysis. The study involved four parts:(1) foliar litter decomposition and controlling factors across forest types and stand age classes; (2) effects of litter initial decomposition stages and soil organism on litter decomposition; (3) seasonal dynamics and the controlling factors of soil respiration across forest types; (4) variations of soil respiration components in the oak chronosequence. The objective was to determine the spatial variations and controlling factors in litter decomposition and soil respiration, aimed at elucidating the carbon cycle dynamics of this region. The main results are as follows:(1) Significant variations were found in the rate of litter decomposition among stands of different tree species but not among stand age classes. The values of decay constant, k, varied from 0.62 in Q. aliena stands to 0.56 in Q. variabilis stands. The reciprocal litter transplant experiment showed that the rate of litter decomposition was on average 5% slower in home-fields than on reciprocal sites. Path analysis identified litter acid-unhydrolyzable residue (AUR) to N ratio, soil pH, soil organic carbon (SOC) and soil microbial biomass carbon (MBC) as most prominent factors controlling the rate of litter decomposition, collectively accounting for 57.8% of the variations; AUR/N had the greatest negative effect on k value. These findings suggest that tree species plays a primary role in affecting forest floor litter decomposition by determining the foliar litter quality, with site environment being a secondary factor contributing to the local variations in foliar litter decomposition in this temperate forest ecosystem.(2) Significant variations were found in the rate of litter decomposition among different treatments (fresh litter, decomposed litter and the mixture of fresh and decomposed litter). The faster rate of foliar litter decomposition in the fresh litter than in decomposed litter could well be explained by higher litter quality. Linear-regression analysis identified litter initial nitrogen concentration (%N), litter carbon to nitrogen ratio(C/NLitter) and litter ash content (%Ash) as the main factors controlling the rate of litter decomposition, collectively accounting for 91.9% of the variations by C/Nitter and %Ash. The experiment of the effect of soil organism on litter decomposition showed that adding naphthalene significantly slowed down the rate of litter decomposition.(3) The seasonal patterns of soil respiration among forest types showed the similar trend with soil temperature, occurring as a unimodal curve. Soil respiration rate correlated significantly (p< 0.001) with soil temperature at 5cm depth. Soil temperature accounted for 87.76%-99.07% of the variations in soil respiration rate; this could be explained by that the relatively high temperature favored the activity of soil organisms. The temperature sensitivity(Q10) of mixed P. armandii IQ. aliena var. acuteserrata forest is significantly higher than that of pure Q. aliena var. acuteserrata forest and Q. variabilis forest, which was related to the fine root biomass (FR), plant seasonality and soil moisture content. Soil nutrition content, soil properties, fine root biomass (FR) and soil microbial biomass are the major factors controlling the annully soil CO2 efflux in this forest ecosystem.(4) The seasonal patterns of soil respiration among the age classes of Q. aliena var. acuteserrata forest (40-yrs,80-yrs,>160-yrs) and different components showed the obvious seasonal dynamics, and occurred as a unimodal curve; this also could be explained by the effect of temperature on the activity of soil organisms. Soil temperature accounted for 87.63%-97.91% of the variations in soil respiration rate. There were no significant variations in soil respiration among different components, which might be explained by the fact that there were no significant differences among the age classes of Q. aliena var. acuteserrata forest in soil properties, forest productivity and micro-environmental conditions. The cumulative soil respiration in treatment with doubled litter was significantly (p< 0.05) higher than that of other components, indicating that more litter can provide more abundant nutrient for the activity of soil microorganisms. There were also correlations of cumulative soil respiration with soil organic carbon (SOC), fine root biomass (FR) and microbial respiration (MR).